Stabilization of NaZn(BH4)3via nanoconfinement in SBA-15 towards enhanced hydrogen release

Article abstract of DOI:10.1039/c2ta00195k

In the present work, the decomposition behaviour of NaZn(BH ₄)₃ nanoconfined in mesoporous SBA-15 has been investigated in detail and compared to bulk NaZn(BH₄)₃ that was ball milled with SBA-15, but not nanoconfined. The successful incorporation of nanoconfined NaZn(BH₄)₃ into mesopores of SBA-15 was confirmed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, ¹¹B nuclear magnetic resonance, nitrogen absorption/desorption isotherms, and Fourier transform infrared spectroscopy measurements. It is demonstrated that the dehydrogenation of the space-confined NaZn(BH₄)₃ is free of emission of boric by-products, and significantly improved hydrogen release kinetics is also achieved, with pure hydrogen release at temperatures ranging from 50 to 150 °C. By the Arrhenius method, the activation energy for the modified NaZn(BH₄)₃ was calculated to be only 38.9 kJ mol^-1, a reduction of 5.3 kJ mol^-1 compared to that of bulk NaZn(BH₄)₃. This work indicates that nanoconfinement within a mesoporous scaffold is a promising approach towards stabilizing unstable metal borohydrides to achieve hydrogen release with high purity.

Full text of DOI:10.1039/c2ta00195k

Journalof
Materials Chemistry A
Materials for energy and sustainability
www.rsc.org/MaterialsA
Volume 1 | Number 2 | 14 January 2013 | Pages 149–416
ISSN 2050-7488
PAPER
Zaiping Guo, Xuebin Yu et al.
Stabilization of NaZn(BH ) via nanoconfinement in SBA-15 towards enhanced
4
3
hydrogen release
2050-7488(2013)1:2;1-2

Journal of
Materials Chemistry A
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PAPER
Stabilization of NaZn(BH ) via nanoconfinement in
4
3
SBA-15 towards enhanced hydrogen release†
Cite this: J. Mater. Chem. A, 2013, 1,
50
2
ab
a
ac
d
b
b
Guanglin Xia, Li Li, Zaiping Guo,* Qinfen Gu, Yanhui Guo, Xuebin Yu,*
a
e
Huakun Liu and Zongwen Liu
In the present work, the decomposition behaviour of NaZn(BH
been investigated in detail and compared to bulk NaZn(BH
nanoconfined. The successful incorporation of nanoconfined NaZn(BH
confirmed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray
4
)
3
nanoconfined in mesoporous SBA-15 has
)
4 3
that was ball milled with SBA-15, but not
into mesopores of SBA-15 was
4 3
)
11
spectroscopy, B nuclear magnetic resonance, nitrogen absorption/desorption isotherms, and Fourier
transform infrared spectroscopy measurements. It is demonstrated that the dehydrogenation of the
space-confined NaZn(BH
4
)
3
is free of emission of boric by-products, and significantly improved hydrogen
ꢀ
release kinetics is also achieved, with pure hydrogen release at temperatures ranging from 50 to 150 C.
Received 5th September 2012
Accepted 26th September 2012
By the Arrhenius method, the activation energy for the modified NaZn(BH
4
)
3
was calculated to be only
ꢁ1
ꢁ1
3
8.9 kJ mol , a reduction of 5.3 kJ mol compared to that of bulk NaZn(BH . This work indicates
4 3
)
DOI: 10.1039/c2ta00195k
that nanoconfinement within a mesoporous scaffold is a promising approach towards stabilizing
www.rsc.org/MaterialsA
unstable metal borohydrides to achieve hydrogen release with high purity.

rst-principles calculations and experiments on the thermody-
1
Introduction
namic stability of metal borohydrides, M(BH ) (M ¼ Li, Na, K,
4
n
Complex metal borohydrides have been regarded as one of the Cu, Mg, Zn, Sc, Zr, and Hf, n ¼ 1–4), demonstrating a viable
most promising systems for hydrogen storage in the light of strategy for the development of new metal borohydrides with
1,2
their high hydrogen capacities. During the past decades, advantageous dehydrogenation properties via regulating the c
intensive research has been performed on the synthesis and of the relevant metal cations. It is also well known, however,
p
9
modication of these compounds so as to enhance their
hydrogen storage properties. The dehydrogenation tempera-
tures of these candidates are still too high for practical low-
that less stable metal borohydrides, due to the presence of
relevant metal cations with high c , release considerable
amounts of diborane by-products in conjunction with the
3
–5
p
temperature applications, because of their thermodynamic liberation of hydrogen, prohibiting their practical application
10
stability due to the strong B–H interactions, which is the main as hydrogen storage materials in fuel cells. Thus, a feasible
6
–8
source for hydrogen release from borohydrides. Recently, a approach is also desirable to achieve a compromise between the
credible correlation between the dehydrogenation temperature favorable release of hydrogen and the thermodynamic stability
and the Pauling electronegativity, c
temperature of metal borohydrides decreases with increasing
p
, (since the decomposition of metal borohydrides.
Recently, a series of dual-cation (M, Zn) compounds (M ¼ Li,
Na) formed by ball milling ZnCl and MBH
(M ¼ Li, Na), along
with MCl (M Li, Na) by-products, i.e., LiZn (BH
NaZn (BH ) , and NaZn(BH ) , in which Zn has a high c of
c
p
of the metal cations) was provided by means of
2
4
¼
2
4 5
) ,
2
+
2
4 5
4 3
p
a
Institute for Superconducting and Electronic Materials, University of Wollongong,
1
.65, were demonstrated experimentally and theoretically to
North Wollongong, Australia. E-mail: zguo@uow.edu.au
ꢀ
undergo thermal decomposition of hydrogen below 100 C,
which is much lower than that for other pure metal borohy-
b
Department of Materials Science, Fudan University, Shanghai, China. E-mail:
yuxuebin@fudan.edu.cn; Tel: +86-21-55664581
c
drides, e.g., LiBH
these dual-cation compounds remarkable potential to be prac-
4 4 4 2 4 2
, NaBH , Mg(BH ) , and Ca(BH ) , giving
School of Mechanical, Materials
& Mechatronics Engineering, University of
Wollongong, NSW 2522, Australia
d
11–15
Australian Synchrotron, 800 Blackburn Rd, Clayton 3168, Australia
tical for hydrogen storage applications.
realized the purication of NaZn(BH
We have recently
using a wet-chemical
e
School of Chemical and Biomolecular Engineering, The University of Sydney, Sydney,
4
)
3
NSW 2006, Australia
Electronic supplementary information (ESI) available: XRD of sediments during
route with separation of the NaCl formed during synthesis,
†
which signicantly increases the practical hydrogen capacity of
preparation of NaZn(BH
4
)
3
–THF solution and pure NaZn(BH
4 3
) decomposed at
16
ꢀ
this system. It has to be borne in mind, however, that as a
result of the instability of Zn-based borohydrides, large
2
1
00 C, and TEM images of NaZn(BH
4
)
3
loaded into SBA-15. See DOI:
0.1039/c2ta00195k
2
50 | J. Mater. Chem. A, 2013, 1, 250–257
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Journal of Materials Chemistry A
amounts of boranes are released during pyrolysis, which is spheres, with a ball-to-powder ratio of 30 : 1, followed by the
substantially harmful to proton exchange membrane fuel cells addition of NaBH (1.135 g) and ZnCl (1.3631 g). To minimize
4
2
(PEMFC), reduces the effective hydrogen capacity, and heating in the vessel, the milling process was paused for 12 min
decreases possibilities for reversibility. Therefore, suppression aer every 30 min milling, with 40 cycles at 300 rpm. Aer the
of boranes released during its decomposition is of vital ball-milling procedure, centrifugation was performed for around
importance for the application of NaZn(BH
storage material.
4
)
3
as a hydrogen 30 min in a sealed tube under Ar atmosphere to separate the
transparent NaZn(BH –THF solution from the precipitated by-
4 3
)
On the other hand, aer several years of exploration, products (e.g., NaCl), which were characterized by X-ray diffrac-
entrapping hydrogen storage materials within the nanopores of tion (XRD) as shown in Fig. S1 in the ESI.† Pure NaZn(BH was
solid mesoporous templates has been found to serve as an then prepared by vacuum-drying NaZn(BH ) –THF solution
4 3
)
4
3
effective tool for enhancing the hydrogen storage properties of (Fig. S2†) under vacuum at room temperature for 48 h.
these materials, in such aspects as improved kinetics and
thermodynamics for the hydrogen release reaction, as well as Preparation of control sample
17–30
effective suppression of volatile gases.
For example, by
A mixture of NaZn(BH ) with SBA-15 as a control sample with a
weight ratio of 1 : 3 was ball milled via a planetary QM-1SP2 for
4
3
conning ammonia borane (AB) in templates such as Li-doped
31
mesoporous carbon (Li-CMK) and metal organic frameworks
1
2
h. The ball-to-powder ratio was 30 : 1 with a milling speed of
50 rpm. Due to the relatively lower onset temperature of
32
(
JUC-32-Y), the dehydrogenation kinetics of ammonia borane
at lower temperature was substantially increased, together with
the total suppression of both borazine and ammonia. Moreover,
the recyclable dehydrogenation of ammonia borane was effec-
tively achieved within graphene oxide-based hybrid nano-
ꢀ
5,12,16
decomposition (approximately 100 C),
the milling proce-
dure was carried out by alternating between 6 min of milling
and 6 min of rest. In order to prevent contamination by air, all
handling and manipulation of the materials were performed in
an argon-lled glove box with a recirculation system to keep the
H O and O levels below 1 ppm.
33
structures. In regards to the pure borohydrides, numerous
encouraging nanostructured or nanoporous materials, such as
carbon nanotubes, carbon bers, mesoporous silica, nano-
porous carbon, and metal–organic frameworks (MOFs) have
been adopted to modify the hydrogen storage properties of
2
2
Preparation of loaded NaZn(BH
4 3
) /SBA-15 composite
In this case, NaZn(BH ) was loaded into SBA-15 by the typical
4
3
LiBH , demonstrating that nanoconnement is an effective way
4
infusion method via capillary action. Typically, 25 drops of
NaZn(BH
to improve the hydrogen desorption/absorption properties of
34–38
4
)
3
–THF solution were added to SBA-15 (ꢂ0.2 g) in the
borohydrides.
4
In particular, LiBH that is nanoconned via
glove box, the mixture was then ultrasonicated for 2 h at around
25 C, and nally, the solvent was removed at room temperature
SBA-15 as a scaffold exhibits enhanced hydrogen desorption
ꢀ
ꢀ
properties, with desorption starting at 150 C, which is more
ꢀ
under vacuum over 48 hours to yield the target compound.
Meanwhile, the weight of the sample before and aer every
treatment was monitored using an electronic balance.
than 200 C reduction compared with the bulk LiBH
4
, con-

rming that SBA-15 is a suitable template for nanoconnement
36
of hydrogen storage materials. Therefore, by taking advantage
of the unique properties of hydrogen storage materials as
nanoparticles, specically based on their high surface-to-
Characterization
volume ratios, leading to shorter diffusion paths for hydrogen Simultaneous thermogravimetric analysis and mass spectrom-
liberation from substrates, SBA-15 was applied as a scaffold to etry (TGA-MS, Netzsch STA 449C) were conducted at room
ꢀ
ꢁ1
investigate the size effect on the desorption properties of temperature, using a heating rate of 2 C min under dynamic
ꢁ
1
NaZn(BH
4
)
3
on the nanoscale.
argon with a purge rate of 50 mL min . Volumetric release for
quantitative measurements of hydrogen desorption from
samples was carried out on a homemade Sievert’s type appa-
ratus under 1 atm Ar atmosphere. Desorption properties of the
samples were also evaluated using Sievert’s volumetric
2
Experimental
Materials
ꢀ
ꢁ1
methods, with a heating rate of 2 C min
under argon
NaBH (98%) and anhydrous ZnCl (99.99%) were purchased
4
2
atmosphere. Before measurement, the system was calibrated by
a LiAlH standard, with the experimental error less than 1%.
from Sigma-Aldrich and used in as-received form without
further purication. Tetrahydrofuran (THF; 99.999%; Sigma-
Aldrich) was further dried over sodium before using and stored
under Ar atmosphere. SBA-15 was obtained from Nanjing
4
Approximately 0.15 g of the sample was loaded, and during
decomposition upon heating, the pressure data and the
temperature data were recorded automatically at every 6 s.
Finally, according to the equation PV ¼ nRT, where R is the gas
constant and V is the intrinsic volume of the equipment, the
moles (n) of the gas released from the sample could be calcu-
lated. Nitrogen absorption/desorption isotherms (Brunauer–
XFNANO Materials Tech Co., Ltd, China, and vacuum-dried at
ꢀ
1
50 C for 6 h in advance before using.
4 3
Preparation of NaZn(BH ) –THF solution
39
For the preparation of NaZn(BH4)3 in THF, anhydrous THF Emmett–Teller (BET) technique) at the temperature of liquid
ꢂ16 mL) was rst transferred in an argon atmosphere to a 50 mL nitrogen via a Quantachrome NOVA 4200e instrument were
ball-milling vessel containing 11 mm diameter stainless steel collected to characterize the pore structure of the samples. The
(
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phase composition of the powders was analyzed by X-ray composite, peaks belonging to NaZn(BH
4
)
3
are present, indi-
diffraction (D8 Advance, Bruker AXS) with Cu Ka radiation. cating that only a physical mixture was produced during the
Amorphous tape was used to prevent any possible reactions ball-milling process. In contrast to the ball-milled composite,
between the sample and air during the XRD measurement. peaks indexed to NaZn(BH4)3 are absent from the loaded
Fourier transform infrared (FTIR, Magna-IR 550 II, Nicolet) NaZn(BH
analysis was conducted to determine the chemical bonding. nanoconnement of NaZn(BH
During the FTIR measurements (KBr pellets), samples were indicating the superior dispersion of NaZn(BH
loaded into a closed tube with KBr for measurement in the pores in comparison to the control sample. In the search for
4
)
3
/SBA-15 sample, which can be attributed to the
inside the pores of SBA-15,
in the meso-
4 3
)
4 3
)
1
1
4 3
argon-lled glove box. Solid-state B nuclear magnetic reso- further evidence for the effective inltration of NaZn(BH ) into
nance (NMR, DSX 300) results were collected using a Doty cross- the pores of SBA-15, N2 adsorption/desorption analysis was
polarization magic angle spinning (CP-MAS) probe with no employed to verify the specic surface area together with the
probe background. All those solid samples were spun at 12 kHz, cumulative pore volume of the mesoporous SBA-15. As we can
using 4 mm ZrO
2
rotors lled up in a puried argon atmosphere see from Fig. 2 and Table 1, both the BET surface area and the
glove box. A 0.55 ms single-pulse excitation was employed, with a cumulative pore volume of SBA-15 aer encapsulation of
2
ꢁ1
3
ꢁ1
repetition time of 1.5 s. The morphology of the samples was NaZn(BH
evaluated using a eld emission scanning electron microscope substantially reduced, concurrent with a signicant reduction
FE-SEM, JEOL 7500FA, Tokyo, Japan) and a transmission of the intensity of the BJH pore size distribution, in comparison
4
)
3
, 13.6 m
g
and 0.03 cm g , respectively, are
(
electron microscope (TEM, JEOL 2011 F, Tokyo, Japan). High- to SBA-15 without loading, providing further conrmation of
resolution X-ray diffraction data were collected on the powder successful encapsulation of NaZn(BH ) in the mesopores of
4
3
beamline of the Australian synchrotron using a Mythen SBA-15.
detector. Owing to the amorphous state of nanoconnement of
NaZn(BH into SBA-15 on the basis of the XRD results, both
FTIR and NMR spectra were collected to further characterize the
NaZn(BH inside the pores of SBA-15. In addition to the peak
4 3
)
3
Results and discussion
4 3
)
ꢁ
1
Sample determination
at ꢂ1100 cm assigned to the vibration of the Si–O–Si moiety
as shown in the FTIR spectrum (Fig. 3), peaks of B–H bonds at
4 3
In order to conrm the successful inltration of NaZn(BH )
into mesoporous SBA-15, XRD, FTIR, NMR, BET, SEM, and TEM
measurements on pure SBA-15, and the loaded and ball-milled
ꢁ1
1
2
120 cm , attributed to bending modes, and in the region of
ꢁ
1
200–2400 cm , also attributed to bending modes, which were
, were
into SBA-15, sug-
exists inside the pores of SBA-15 as ne
crystallites. Moreover, for the as-prepared and nanoconned
NaZn(BH , the absence of characteristic peaks of C–H bonds
Fig. S3†) in the range of 2800–3000 cm
quite analogous to those of the as-prepared NaZn(BH
observed aer the inltration of NaZn(BH
gesting that NaZn(BH
4 3
)
NaZn(BH
4
)
3
/SBA-15 composites were conducted. From the
incorporation, a
4 3
)
increased weight of SBA-15 aer NaZn(BH )
weight loading rate of 24 wt% is deduced for the loaded
composite. In the XRD results, as shown in Fig. 1, a broad peak
was present in the range of 2q from 20 to 40 for all samples
involving SBA-15, as a consequence of the components for
amorphous silica that are present in SBA-15. With respect to the
control sample of the ball-milled NaZn(BH ) /SBA-15
4 3
4 3
)
ꢀ
ꢀ
4 3
)
ꢁ
1
(
indicates the
complete removal of THF. It is interesting to see, however, that
the intensity of B–H vibrations of nanoconned NaZn(BH4)3
was signicantly decreased compared with pure NaZn(BH ) ,
4 3
4
3
Fig. 1 X-ray diffraction patterns of pure SBA-15, ball-milled NaZn(BH
4
)
3
/SBA-15
Fig. 2 Pore size distributions for (a) the mesoporous SBA-15 and (b) the loaded
NaZn(BH /SBA-15 composite from the BET results. The inset shows the corre-
sponding nitrogen adsorption and desorption isotherms.
composite, and loaded NaZn(BH
4
)
3
/SBA-15 along with its products after dehy-
4 3
)
ꢀ
drogenation to 200 C.
2
52 | J. Mater. Chem. A, 2013, 1, 250–257
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Table 1 BET and Langmuir surface areas, and total pore volume of the meso- boron in the region of 16 to ꢁ20 ppm appears in the NMR
porous SBA-15 with and without NaZn(BH
4
)
3
loading
ꢁ1
spectra of nanoconned NaZn(BH ) in SBA-15, originating
from possible decomposition during solid state NMR
measurements, owing to its relatively much lower dehydroge-
4
3
2
Surface area (m g
)
Total pore
volume (cm g
3
ꢁ1
Samples
BET
Langmuir
)
nation temperature in contrast to pure NaZn(BH
cussed below).
4 3
) (as dis-
Mesoporous SBA-15
Loaded NaZn(BH ) /SBA-15
4 3
274.6
13.6
442.1
22.1
0.4596
0.0293
The surface morphology changes for the nanoconned
NaZn(BH before and aer dehydrogenation were obtained
4 3
)
through SEM and high resolution TEM (HRTEM) images. As
shown in Fig. 4b, aer vacuum drying, agglomeration due to the
thus-crystallized NaZn(BH ) is obviously absent on the surfaces
4
3
of SBA-15, while the Si map agrees well with the structure of the
SBA-15 and the Zn map corresponds well with the Si map from
the energy dispersive spectroscopy (EDS) elemental maps for
the loaded compound (Fig. 4d–f), suggesting good dispersion of
NaZn(BH
show the distribution of NaZn(BH )
4
) inside the pores of SBA-15. The TEM images in Fig. 5
in the channel pores of the
4 3
samples before and aer dehydrogenation. The characteristic
hexagonal arrays of uniform channels with the typical honey-
comb appearance of SBA-15 material (Fig. S4†), presenting a
channel diameter of around 6.5 nm, were denitely observed, in
accordance with the average pore diameter of approximately
11
Fig. 3 FTIR (left) and B MAS solid-state NMR (right) spectra of SBA-15, ball-
milled NaZn(BH /SBA-15 composite, and loaded NaZn(BH /SBA-15.
4
)
3
4 3
)
while there was also increased intensity of the O–H vibrations
compared with pure SBA-15, which presents only a slight peak
due to O–H vibrations. This is indicative of weakened B–H
bonds and a possible attractive interaction between O of SBA-15
and H of the BH
NaZn(BH into SBA-15. Furthermore,
revealed that typical resonances of BH4 groups centered at
42 ppm and ꢁ45.2 ppm for the synthesized NaZn(BH ) could
4
group aer incorporation of nanoscale
1
1
4
)
3
B NMR spectra
ꢁ
1
ꢁ
4
3
+
2+
be assigned to BH groups related to Na and Zn , respectively,
4
16
in agreement with the literature, while the resonances of
ꢁ1
BH
a roughly 1.6 ppm shi to the downeld region, which was also
observed for a LiBH –MgBH mixture aer inltration into
4
4 3
groups of NaZn(BH ) aer inltration into SBA-15 have
4
4
40
carbon pores. This result indicates the increased electron
density of boron in NaZn(BH ) due to the different chemical
4
3
environment induced upon nanoconnement compared to
pure NaZn(BH ) . The downeld shi of BH groups in the
4
3
4
NMR spectra can be mainly attributed to (1) the attractive
interaction between O of SBA-15 and H of the BH moiety, as
indicated by the FTIR spectra, which will weaken the electron
withdrawal by H from B in the BH group and, vice versa, result
4
4
in increased electron density of atomic B; and (2) the possible
shielding effect of lone-pair electrons of O atoms from SBA-15
on the resonances of boron in the NaZn(BH ) inside the pores
4
3
4 3
Fig. 4 SEM images of (a) pure SBA-15 for comparison, (b) loaded NaZn(BH ) /
of SBA-15, at least to a certain degree, indirectly demonstrating
the successful inltration of NaZn(BH into SBA-15. In addi-
ꢀ
SBA-15 after vacuum drying, (c) the products after dehydrogenation to 200 C,
and the corresponding EDS maps of Si (e) and Zn (f), corresponding to image (d),
4 3
)
tion, a low peak with a broad shoulder pertaining to amorphous collected before dehydrogenation.
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NaZn(BH
4
)
3
, the NaZn(BH
4
)
3
loaded into SBA-15, and the ball-
milled NaZn(BH ) /SBA-15 composite as a control sample was
4
3
conducted by employing TGA and MS in tandem with volu-
metric measurements, as shown in Fig. 6 and 7a. The decom-
position of the as-prepared NaZn(BH
4
)
3
in the temperature
ꢀ
16
range of 80–200 C is similar to what was previously reported,
with a weight loss of ꢂ29 wt%, accompanied by the release of a
3 2 6
multitude of borane families, e.g., BH and B H , which is
further veried by the much lower amount of hydrogen release
in comparison with its theoretical content, with only 1.35 mol of
gas liberated during the thermolysis procedure, according to
the volumetric results expressed with respect to the content of
NaZn(BH
cating a signicantly reduced amount of effective hydrogen. In
the case of the ball-milled mixture of NaZn(BH with SBA-15,
as most NaZn(BH is present outside of the SBA-15, instead of
4 3
) in the sample under the same conditions, indi-
4 3
)
4 3
)
being incorporated into the nanopores of SBA-15, it undergoes a
decomposition process that is comparable with that of pure
NaZn(BH ) , exhibiting a 24.1 wt% weight loss, which is much
4
3
more than its theoretical hydrogen storage capacity (9.1 wt%),
but with only 1.4 mol of released gas from volumetric results. In
contrast, from the MS spectra of NaZn(BH
SBA-15, only hydrogen was detected over the whole thermolysis
2 6
process, coupled with a total suppression of BH and/or B H .
4 3
) nanoconned in
3
In addition, the onset decomposition temperature of
ꢀ
NaZn(BH ) inltrated into SBA-15 decreased to around 50 C,
4
3
ꢀ
along with one peak at 112 C, which is slightly lower than those
ꢀ
of pure NaZn(BH4)3 and the control sample (ꢂ120 C).
Furthermore, the weight loss (6 wt%) is in good agreement with
the volumetric results, with around 3.98 mol of gas released
upon heating, accounting for 99% of theoretical hydrogen
capacity, which further conrms the complete repression of
release of boric by-products. The weight loss process for dehy-
4 3
Fig. 5 HRTEM images at different magnifications of the loaded NaZn(BH ) /
SBA-15 before (a and b) and after (c and d) dehydrogenation; TEM image before
dehydrogenation (e) and the corresponding EDS maps of Si (f), Zn (g), and Na (h).
ꢀ
drogenation ends before 150 C, in concurrence with the MS
6
nm derived from the BET results. Additionally, dark domains
and spots inside the mesopores of SBA-15, resulting from the
nanoconned NaZn(BH and/or its decomposed products,
spectra, which is much lower than that of pure NaZn(BH ) ,
4
3
4 3
)
denoted by arrows, are apparent, with EDS elemental maps of Si
coinciding with those of Zn and Na, providing strong conr-
mation for the incorporation of NaZn(BH4)3 into SBA-15.
Moreover, it should be pointed out that the pore structure of the
ꢀ
loaded samples, aer heat treatment to 200 C (Fig. 5c and d),
remains intact, which agrees well with the SEM images of the
loaded samples (Fig. 4b and c) regardless of whether or not they
had experienced heat treatment, demonstrating that SBA-15 can
effectively maintain its nanoconnement effects on the
NaZn(BH ) inside its pores throughout the process of hydrogen
4
3
desorption.
Based on the above-mentioned characterization, we can
therefore conclude that NaZn(BH was indeed incorporated
4 3
)
into SBA-15 in the nanoscale form and/or blocked the pores of
SBA-15 by dint of infusion through the capillary effect.
Hydrogen desorption properties
Fig. 6 TGA (left) and MS (right) results for pure NaZn(BH (black line), ball-
4
)
3
milled NaZn(BH /SBA-15 (red line), and loaded NaZn(BH ) /SBA-15 (blue line),
4
)
3
4 3
To explore the effects of SBA-15 as a scaffold on the hydrogen
storage properties of nanoconned NaZn(BH ) , a comparison
4 3
ꢀ
ꢁ1
with a heating rate of 2 C min under dynamic argon atmosphere. The right axis
of the TGA chart gives the amount of weight loss relative to the mass of
4 3
of the thermal decomposition performance of the as-prepared NaZn(BH ) only.
2
54 | J. Mater. Chem. A, 2013, 1, 250–257
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Journal of Materials Chemistry A
achieve a pure hydrogen release, in tandem with enhanced
hydrogen release kinetics, making NaZn(BH4)3 a promising
hydrogen storage material for practical application.
Decomposition mechanism
In the light of the above discussion, the connement of
NaZn(BH ) in SBA-15 on the nanoscale, giving rise to shorter
4 3
diffusion paths for hydrogen evolution, is the main factor
Fig. 7 (a) Volumetric gas release measurements for pure NaZn(BH
4
)
3
(black line), responsible for enhancing the hydrogen desorption properties,
ball milled NaZn(BH
line), with a heating rate of 2 C min under 1 atm argon, expressed with respect
to the content of NaZn(BH only, and (b) isothermal volumetric results for gas
4
)
3
/SBA-15 (red line) and loaded NaZn(BH
4
)
3
/SBA-15 (blue similar to what has been observed in other nanoconned
ꢀ
ꢁ1
17–35
hydrogen storage materials.
Further investigation of the
4 3
)
effects of SBA-15 as a scaffold on the decomposition mechanism
ꢀ
ꢀ
ꢀ
release from the loaded NaZn(BH
4
)
3
/SBA-15 composite at 80 C, 90 C, 100 C,
ꢀ
ꢀ
was conducted, in order to unravel the mechanism for
and 110 C, and for pure NaZn(BH
4
)
3
at 110 C. The inset shows a comparison of
the Arrhenius plots of the temperature-dependent rate data of the loaded improvement of the thermodynamics of hydrogen release. The
NaZn(BH
4
)
3
/SBA-15 composite (C) and the pure NaZn(BH
4
)
3
(B).
XRD pattern (Fig. S6†) of NaZn(BH
)
aer decomposition to
4
3
ꢀ
200 C demonstrates the formation of Zn and NaBH , without
4
the appearance of any boron peaks due to its amorphous
structure, which corresponds well with the results of a previous
4 3
clearly demonstrating that the nanoconnement of NaZn(BH )
facilitates the kinetics of hydrogen release.
16
report. Only peaks indexed to Zn are observed, however, for
the dehydrogenated product of NaZn(BH loaded into SBA-15,
with no presence of NaBH (Fig. 1). As apparent BH
corresponding to NaBH
4 3
)
Better information on the decomposition properties of
ꢁ
1
4
4
groups
nanoconned and bulk NaZn(BH ) was gained via isothermal
4
3
4
centered at ꢁ44.2 ppm were detected
dehydrogenation at different temperatures through the method
of volumetric gas release measurements, as shown in Fig. 7b
and S5,† to investigate the dehydrogenation kinetics. As
expected, the hydrogen desorption kinetics increases as the
relative operating temperature increases. The results reveal that
in the NMR spectra, in addition to the B–H bonds in both
bending and stretching modes, as witnessed by the FTIR spectra
(
Fig. 8), the disappearance of NaBH from the XRD spectra may
4
have resulted from its uniform dispersion in the mesoporous
SBA-15 due to its amorphous and/or nanophase structure. This
is directly conrmed by the TEM results (Fig. 5c and d), which
exhibit nanocrystals of products inside the mesopores of SBA-15
aer hydrogen desorption. Simultaneously, the intensity of O–H
interaction vibrations was decreased with the decreasing
intensity of B–H bond vibrations, suggesting that the presence
of O–H interactions may promote hydrogen release from
NaZn(BH ) . With regard to the NMR spectra of the dehydro-
hydrogen release from the nanoconned NaZn(BH
4
)
3
proceeds
ꢀ
slowly at the temperature of 80 C, evolving about 2 wt%
hydrogen within 250 min. Upon heating, nanoconned
NaZn(BH ) could release around 3 wt% hydrogen in 200 min at
4
3
ꢀ
ꢀ
9
0 C and slightly more than 4 wt% within 150 min at 100 C.
Specically, 5.7 wt% hydrogen, accounting for 95% of its
theoretical hydrogen capacity, was released from NaZn(BH
aer encapsulation in SBA-15 in only 90 min, while merely 0.5
mol gas was released from pure NaZn(BH at the same
4 3
)
4
3
genated products, the disappearance of peaks assigned to the
4 3
)
2
+
BH
4
4 3
group linked to Zn in NaZn(BH ) with and without
temperature, even with time extension to 300 min, which is
much less than the hydrogen release capacity of the nano-
ꢀ
conned NaZn(BH4)3 even at 80 C. Clearly, a signicant
4 3
enhancement of the hydrogen release kinetics of NaZn(BH )
aer encapsulation into SBA-15 is achieved due to the forma-
tion of nanocrystals, resulting in shorter diffusion paths for
hydrogen spillover from the substrate, which lowers the kinetic
dehydrogenation barrier. In order to quantitatively characterize
the kinetic properties, Arrhenius plots of ln k were collected
ꢁ
1
(
where k is the reaction rate constant (s )) versus 1/T (where T is
the absolute temperature for relative hydrogen release), as
shown in the inset of Fig. 7b. The plots exhibit good linearity for
the nanoconned NaZn(BH
data points, the apparent activation energy E
4 3
release from nanoconned and bulk NaZn(BH )
4
)
3
. By conducting a linear t of the
for hydrogen
, by virtue of
a
Arrhenius equation (k ¼ Aexp(ꢁE
a
/(RT))), can be calculated to be
only 38.9 kJ mol and 44.2 kJ mol , respectively. This directly
ꢁ1
ꢁ1
conrms the enhancement of kinetics for NaZn(BH ) conned
4
3
in mesopores of SBA-15. To the best of our knowledge, this is
the rst example of successful stabilization of a Zn-based metal
borohydride via the nanoconnement technique so as to to 200 C under 1 atm argon.
11
Fig. 8
B MAS solid-state NMR (left) and FTIR (right) spectra of (a) pure
and (b) loaded NaZn(BH /SBA-15 composite after decomposition
NaZn(BH
4
)
3
4 3
)
ꢀ
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inltration into SBA-15 conrms the complete decomposition
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4
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This work was partially supported by the National Natural
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